The two machines in question have different MAC addresses for eth0. However, when I apply the following netplan file, I end up with the same MAC address on both machines, which, of course, is kind of a problem. I'm using Ubuntu 18.04.1 on ARM.
network:
version: 2
renderer: networkd
ethernets:
eth0:
dhcp4: no
bridges:
br0:
interfaces: [eth0]
dhcp4: true